1. Field of the Invention
This invention relates to an apparatus for determining a failure of an oxygen concentration sensor that is arranged at a location downstream of a catalyst in the exhaust system of an internal combustion engine and detects a concentration of oxygen in exhaust gases.
2. Description of the Prior Art
Conventionally, an apparatus for determining a failure of an oxygen concentration sensor that is arranged in the exhaust system of an internal combustion engine has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 4-36651, in which a failure of the oxygen concentration sensor (hereinafter referred to as xe2x80x9cthe O2 sensorxe2x80x9d) is determined in the following manner: First, the output (hereinafter referred to as xe2x80x9cthe sensor outputxe2x80x9d) from the O2 sensor is sampled before, during, and after a fuel cut-off operation (hereinafter referred to as xe2x80x9cthe F/C operationxe2x80x9d) in which supply of fuel to the engine is cut off. When a gradient of fall (response) of the sensor output immediately after the start of the F/C operation is smaller than a predetermined reference value, when a gradient of rise of the sensor output immediately after the end of the F/C operation is smaller than a predetermined reference value, or when the difference between the minimum value of the sensor output during the F/C operation and the maximum value of the sensor output after the termination of the F/C operation is smaller than a predetermined value, it is determined that the O2 sensor has failed (been degraded).
According to the above conventional apparatus, for instance, if the engine is running in idle immediately after termination of the F/C operation, the amount of exhaust gases supplied to the O2 sensor is very small, so that the gradient of rise of the sensor output can be smaller than the above predetermined reference value, causing erroneous determination that the O2 sensor has failed although actually it has not.
Further, if the apparatus is applied to an O2 sensor arranged at a location downstream of a catalyst in an exhaust pipe, the following inconvenience arises: During an F/C operation, oxygen is stored in the catalyst, so that immediately after termination of the F/C operation, the concentration of oxygen in exhaust gases having passed through the catalyst is high. As a result, the gradient of rise of the sensor output can be smaller than the predetermined reference value, or the above difference between the minimum value of the sensor output during the F/C operation and the maximum value of the sensor output after the termination of the F/C operation can be smaller than the predetermined value, causing the same erroneous determination as mentioned above.
It is an object of the invention to provide an apparatus for determining a failure of an oxygen concentration sensor, which is capable of accurately and promptly determining the failure of the sensor.
To attain the above object, the invention provides an apparatus for determining a failure of an oxygen concentration sensor of an internal combustion engine that includes an exhaust system, and a catalyst arranged in the exhaust system, and is capable of carrying out a fuel cut-off operation, the oxygen concentration sensor being arranged in the exhaust system at a location downstream of the catalyst, for generating a signal indicative of a sensed concentration of oxygen in exhaust gases.
The apparatus according to the first aspect of the invention is characterized by comprising:
oxygen concentration change-determining means for determining whether or not a signal value of the signal from the oxygen concentration sensor has crossed a predetermined threshold value after termination of the fuel cut-off operation;
exhaust gas amount calculation means for calculating an integrated amount of the exhaust gases exhausted from the engine after the termination of the fuel cut-off operation; and
failure determination means for determining that the oxygen concentration sensor has failed when the oxygen concentration change-determining means determines that the signal value has not crossed the predetermined threshold value, in the case of the integrated amount of the exhaust gases having reached a predetermined value.
According to this apparatus, it is determined whether or not the signal value of the signal from the oxygen concentration sensor has crossed the predetermined threshold value after termination of a fuel cut-off operation, and when the integrated amount of exhaust gases exhausted after the termination of the fuel cut-off operation has reached the predetermined value, if it is determined that the signal value has not crossed the predetermine threshold value, it is determined that the oxygen concentration sensor has failed. Thus, when the integrated amount of exhaust gases has reached the predetermined value, the determination of a failure of the oxygen concentration sensor is carried out based on the signal value output therefrom. Therefore, even when the amount of exhaust gases is small, it is possible to determine the failure of the oxygen concentration sensor when the exhaust gases are supplied to the catalyst in a sufficient amount, i.e. when the catalyst has recovered from its excess oxygen-storing state in which too much oxygen is stored, by full consumption of the stored oxygen by the exhaust gases. This enables accurate determination of a failure of the oxygen concentration sensor.
Preferably, the apparatus further includes failure determination-suspending means for suspending determination of the failure of the oxygen concentration sensor by the failure determination means, when a predetermined time period has elapsed after the termination of the fuel cut-off operation, on condition that the integrated amount of the exhaust gases has not reached the predetermined value, and at the same time, the oxygen concentration sensor determines that the signal value has not crossed the predetermined threshold value.
According to this preferred embodiment, on condition that the integrated amount of exhaust gases has not reached the predetermined value, and at the same time it is determined that the signal value from the sensor has not crossed the predetermined threshold value, when the predetermined time has passed after the termination of the fuel cut-off operation, i.e. when exhaust gases continue to be supplied in a small amount over a long time period, the determination of a failure of the oxygen concentration sensor by the failure determination means is suspended. This makes it possible to prevent an increase in an error in the integrated amount of exhaust gases resulting from a long-term integration of a small amount of the supply of exhaust gases thereto, and thereby prevent an erroneous determination of a failure of the oxygen concentration sensor due to an erroneous value of the integrated amount of exhaust gases. Further, since the determination of a failure of the oxygen sensor is suspended with reference to the predetermined time period elapsed after the termination of the fuel cut-off operation, it is possible to properly set the timing of discontinuation of the oxygen concentration sensor failure determination process.
Preferably, the apparatus further includes air-fuel mixture-enriching means that makes an air-fuel mixture supplied to the internal combustion engine immediately after the termination of the fuel cut-off operation, richer than a stoichiometric air-fuel ratio.
According to this preferred embodiment, an air-fuel mixture richer than the stoichiometric air-fuel ratio is supplied to the internal combustion engine immediately after termination of an fuel cut-off operation, whereby exhaust gases containing a high concentration of HC and a high concentration of CO are supplied to the catalyst. This makes it possible to positively cause consumption of oxygen stored in the catalyst during the fuel cut-off operation, thereby causing the catalyst to promptly recover from a state in which an excessive amount of oxygen is stored therein. As a result, exhaust gases with a rich air-fuel ratio, i.e. having a low concentration of oxygen can be supplied to the oxygen concentration sensor soon after termination of the fuel cut-off operation, whereby it is possible to promptly determine a failure of the oxygen concentration sensor, as described above.
To attain the above object, according to a second aspect of the invention, there is provided an apparatus for determining a failure of an oxygen concentration sensor of an internal combustion engine that includes an exhaust system, and a catalyst arranged in the exhaust system, and is capable of carrying out a fuel cut-off operation, the oxygen concentration sensor being arranged in the exhaust system at a location downstream of the catalyst, for generating a signal indicative of a sensed concentration of oxygen in exhaust gases.
The apparatus according to the second aspect of the invention is characterized by comprising:
timer means for measuring a time period elapsed after termination of the fuel cut-off operation;
oxygen concentration change-determining means for determining whether or not a signal value of the signal from the oxygen concentration sensor has crossed a predetermined threshold value after the termination of the fuel cut-off operation;
exhaust gas amount calculation means for calculating an integrated amount of the exhaust gases exhausted from the engine after the termination of the fuel cut-off operation;
failure determination means for determining the failure of the oxygen concentration sensor based on a result of determination of the oxygen concentration change-determining means; and
failure determination-suspending means for suspending the determination of the failure of the oxygen concentration sensor by the failure determination means, when the time period measured by the timer means has reached a predetermined time period, on condition that the oxygen concentration sensor determines that the signal value has not crossed the predetermined threshold value, and at the same time the integrated amount of the exhaust gases has not reached a predetermined value.
According to this apparatus, it is determined whether or not the signal value of the signal from the oxygen concentration sensor has crossed the predetermined threshold value after termination of a fuel cut-off operation, and the integrated amount of exhaust gases exhausted after the termination of the fuel cut-off operation is calculated. Then, a failure of the oxygen concentration sensor is determined based on a result of determination of the oxygen concentration change-determining means. Further, when a time period elapsed after the termination of the fuel cut-off operation has reached a predetermined time period, if the signal value output from the oxygen concentration sensor has not crossed the predetermined threshold value, and at the same time the integrated amount of the exhaust gases has not reached the predetermined value, failure determination on the oxygen concentration sensor is suspended. Thus, when the predetermined time period has elapsed after termination of a fuel cut-off operation, if exhaust gases are not sufficiently supplied to the oxygen concentration sensor, differently from the conventional oxygen concentration sensor, determination of a failure of the sensor is not carried out unless the signal value from the oxygen concentration sensor has crossed the predetermined threshold value. Hence, it is possible to prevent erroneous determination of a failure of the sensor, which can be caused by an insufficient supply of exhaust gases to the oxygen concentration sensor.
Preferably, the apparatus further includes air-fuel mixture-enriching means that makes an air-fuel mixture which is supplied to the internal combustion engine immediately after the termination of the fuel cut-off operation, richer than a stoichiometric air-fuel ratio.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.